1. Field of the Invention
The invention relates to an infinitely variable transmission for a device, for instance a vehicle, with a combustion engine, provided with an endless transmission member which has been looped over two pulleys, each having a running groove situated between conical pulley halves, while at least one conical half of each pulley is axially movable through an associated hydraulic cylinder, also provided with a first control valve for regulating the fluid pressure in the cylinder of the driven pulley and in the supply line to a second control valve which is intended for adjustment of the transmission ratio by regulation of the amount of fluid supplied to and discharged from the cylinder of the driving pulley.
2. Description of the Related Art
In such a device, which has earlier been proposed and described in the Netherlands patent application 7414914, the tension in the endless transmission member (drive belt) is controlled irrespective of the setting of the transmission ratio by means of the first control valve. To do so, the first control valve reduces the fluid pressure built up by a pump, and this reduced fluid pressure exists in one of the operating cylinders, for the purpose of setting the tension of the drive belt. The other operating cylinder is supplied with fluid by a second control valve which transmits the reduced fluid pressure to or from that operating cylinder in order that, depending on a control signal, the transmission ratio may be set through a specific position of that operating cylinder.
The tension in the endless transmission member must be sufficient under all conditions to transmit the engine torque without any slip. As this tension is, in the equilibrium situation, determined in the driven pulley, it is essential that the fluid pressure which governs the pinching force between the pulley halves has at all times a high enough value under divergent conditions of load, contributing factors being the set transmission ratio and the speed of the driving pulley, among other things. Accordingly, the control system provides a certain fluid pressure by means of the valves. However, for slipfree operation upon extreme changes in load, for instance upon fierce acceleration when the device is used for the propulsion of a vehicle, this fluid pressure must be higher than under the normal, more or less constant load conditions. To ensure that no slip occurs upon extreme changes, it has till now been common practice to set the fluid pressure and hence the pinching force in the driven pulley to a higher value than is required with the normal, more or less constant load. In consequence, the tension in the transmission member (drive belt) is also greater, which may have an adverse effect on, for instance, the efficiency and the useful life.